Several different methods have been developed for controlling vehicle lighting systems. The earliest models were strictly mechanical switches packaged in a single housing. This method can be difficult to actuate due to mechanical forces and sequences and the failure rate is high due to constant actuation and mechanical wear of the switch components. Protection circuitry consisted of a bi-metal switch that would break the battery line when a current overload condition was present. When an over-current mode existed, this solution would not resolve the problem quickly enough, causing excessive heating and possible damage to other components.
Another method was the use of solid state devices, which improved ease of use and increased the product life. While solid state devices provide rapid response for shutting down in an over-current condition, they were vulnerable to over-voltages which could cause the devices to fail.